Conservation of outdoor plaster finishesin an architectural complex: The case of the Sacro Monte di Varallo Sesia

Conservation of outdoor historical finishes represents a difficult issue, due to their aesthetical and functional roles. This is particularly the case of the Sacro Monte di Varallo Sesia, in which the protection of the external decoration is preparatory to the protection of the masonry, including the complex indoor decoration. Moreover, the appearance of the external decoration represents itself a remarkable conservation issue, influencing the overall aspect of the Chapels of the Sacro Monte complex. Besides further characterization of existing plasters, this study also deals with plaster laboratory formulation, in order to plan a possible intervention of re-proposition. This last topic is strongly related to color feature analysis on original plaster and to the selection of current available pigments to obtain the most similar color to the in situ plaster ones. These aspects guarantee a chemical and technological compatibility which have an impact on the final appearance. However, characterization and reintegration are not sufficient to fully pursue a conservation plan. It is well known that reintegration intervention should be conducted only when lacunae are too extensive, but the goal of preserving the original finishes as much as possible, will be achieved through the use of consolidants and protectives. For these reasons, we also mention the application of a new protective coating on plaster sample